Silicon dioxide. Illustration showing the basic tetrahedral co-ordination and crystalline structure of silicon dioxide (chemical formula: SiO2), also known as silica. The atoms are colour-coded: silicon (yellow) and oxygen (red). Each silicon atom has tetrahedral co-ordination with four oxygen atoms, but in the overall quartz crystal structure (not shown) each oxygen atom is shared with another silicon atom. Silicon dioxide, most commonly found as quartz, has a wide range of applications.